1) Field of the Invention
This invention relates generally to the removal of photoresist from a semiconductor wafer and more particularly to a process for removing photoresist and a polymer using an aqueous oxidant bath (e.g., aqueous peroxydisulfate in HCl) and simultaneous UV laser irradiation.
2) Description of the Prior Art
The semiconductor industry is driven by the constant need to make smaller and denser devices with greater complexity. Advances in photolithography drive semiconductor miniaturization. Tough challenges to microlithography and, consequently, in photoresist stripping are created by this miniaturization. Present fabrication technology is capable of dealing with device feature sizes of 0.35 .mu.m, but the critical dimensions of ICs are expected to approach 0.18 .mu.m by the year 2000. The anticipated reduction in dimensions requires considerable changes in manufacturing technologies, including the stripping process.
Two main photoresist stripping methods are used in the semiconductor fabrication industry: (1) Dry stripping uses mostly plasma, and to a lesser extent, O.sub.3, O.sub.3 /N.sub.2 O, or UV/O.sub.3 -based stripping; and (2) Wet stripping uses acids such as H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 (Piranha) or organic solvents.
Dry stripping using reactive plasma ashing, suffers from drawbacks from incomplete removal of photoresist and resist popping. Plasma ashing is also associated with several types of damage mechanisms introduced or aggravated by the plasma. These damage mechanisms are due to charges, currents, electric-field-induced UV radiation, contamination (such as alkali ions, heavy metals, and particulates), and elevated temperatures. Resist removal is often incomplete, especially after "tough" processing such HDP and RIE, where the photoresist undergoes chemical and physical changes and forms hard sidewalls. Since plasma ashing often leaves residues, a wet strip must follow to complete the stripping process. In many cases, to avoid alkali and heavy metals contamination, the plasma ashing is stopped before the endpoint, and the wafer is transferred to a wet bath.
For the second stripping method, the wet-stripping method, disadvantages include solution concentrations that change with the number of wafers being stripped, thus affecting stripping quality and throughput; accumulation of contaminants in the baths, which drastically affects yield; and severely corrosive and toxic solutions that impose high handling and disposal costs and create serious safety considerations. Other wet-stripping problems are due to mass transport and surface tension associated with the solutions. For deep sub micron technologies, the solutions cannot circulate and tend to accumulate within the patterned structure. This situation is intolerable, as it contaminates the wafer with foreign materials that can lead to drastic yield losses. All of these wet-stripping problems will become even more critical for 300 + mm wafers.
The inventor has found that a major problem with conventional photoresist strip processes is the incomplete removal of photoresist and hydrocarbon residual polymers, formed during the dry etching of silicon nitride. These polymers form over the photoresist and in narrow spaces and holes. These polymers create defects if the polymers and photoresist are not completely removed before the next process step. Therefore there is a need for an improved photoresist and polymer removal process that can remove the both the tough polymer residues formed during nitride dry etching and the photoresist.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 5,035,918(Vyas): Non-Flammmable And Strippable Plating Resist And Method Of Using Same--shows a method of removing resist using a controlled excimer laser pulse to remove such resist within the areas by an ablation process or a "patch blow off" energy efficient process. U.S. Pat. No. 5,381,807(Lee) shows a method of striping resists and polymers using hydroxylamine and alkanolamine. U.S. Pat. No. 5,630,904(Aoyama) shows a method of striping resists using an organocarboxylic. U.S. Pat. No. 5,114,834(Nachshon) shows a PR removal process using UV ablation. U.S. Pat. No. 4,718,974(Minaee) shows a photoresist stripping apparatus using a UV lamp.
The article "Laser, Dry And Plasmaless Photoresist Removal", by Livshits et al., Solid State Technology, July 1997, pp. 197-202. discusses laser ablation with a plasmaless dry chemistry.
However, these photoresist strip methods can be further improved.